Tape printing apparatus of the type with which the present invention is concerned are known. They operate with a supply of tape arranged to receive a printed image, and a means for transferring the image onto the tape. In one embodiment, a tape holding case or a cassette holds a supply of image receiving tape and a supply of an image transfer ribbon. The image receiving tape and the image transfer ribbon (ink ribbon) are passed in overlap through a printing zone of the tape printing apparatus. A tape printing apparatus operating with a tape holding case of this type is described, for example, in U.S. Pat. No. 4,815,871 to McGourty et al. ("the McGourty patent"). Other printing apparatus have been made in which letters are transferred to an image receiving tape by means of a dry lettering or dry film impression process. In all of these printing apparatus, the construction of the image receiving tape is substantially the same; that is, it comprises an upper layer adapted to receive a printed image and secured to a releasable backing layer by means of a layer of adhesive.
In a further tape printing apparatus as described, for example, in U.S. Pat. No. 5,424,757 to Thom, a tape holding case holds a supply of image receiving tape and a supply of an image transfer ribbon, the image receiving tape having the same construction as described in the foregoing with reference to the McGourty patent.
To perform a printing operation with the known tape printing apparatus, it is necessary to bring a thermal print head and a cooperating platen roll into an operative position in which the image receiving tape rests against the platen roll while the print head rests against the image transfer ribbon or (as a rule) against an ink ribbon. In the operative position, the print head and the platen roll exert a contact pressure against each other which is necessary to effect a printing operation. In order to be able to load a different cassette, means are provided for moving the print head and the platen roll into an inoperative position in which they are relatively spaced. This may be accomplished by moving the print head or the platen roll. This movement may, according to the Thom patent, be coupled to a lid closing the cassette receiving bay of the tape printing apparatus relative to the outside.
A disadvantage of the known tape printing apparatus is that there is contact between the platen roll and the print head in cases where no cassette is loaded and the lid is closed. As a result of the contact force of the print head against the elastic platen roll, the platen roll undergoes a substantially greater deformation than it does with a cassette loaded. A high print quality cannot be expected with a platen roll that is not of a precisely circular-cylindrical configuration. Thus, improvements in this area are needed.